Segment Information - Net Operating Income of Company's Reportable Segments (Details) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2019	Mar. 31, 2018
Segment Reporting Information [Line Items]
Total revenue		$ 143,896	$ 136,358
Operating expenses	[1]	(57,381)	(56,504)
Net operating income (loss)		86,515	79,854
Operating Segments [Member] | Philadelphia CBD [Member]
Segment Reporting Information [Line Items]
Total revenue		65,798	62,602
Operating expenses	[1]	(25,185)	(24,327)
Net operating income (loss)		40,613	38,275
Operating Segments [Member] | Pennsylvania Suburbs [Member]
Segment Reporting Information [Line Items]
Total revenue		35,627	34,882
Operating expenses	[1]	(12,972)	(12,964)
Net operating income (loss)		22,655	21,918
Operating Segments [Member] | Austin, Texas [Member]
Segment Reporting Information [Line Items]
Total revenue	[2]	24,766	8,364
Operating expenses	[1],[2]	(9,076)	(3,523)
Net operating income (loss)	[2]	15,690	4,841
Operating Segments [Member] | Metropolitan DC [Member]
Segment Reporting Information [Line Items]
Total revenue	[3]	13,520	23,059
Operating expenses	[1],[3]	(6,204)	(8,759)
Net operating income (loss)	[3]	7,316	14,300
Operating Segments [Member] | Other [Member]
Segment Reporting Information [Line Items]
Total revenue		3,182	6,141
Operating expenses	[1]	(2,145)	(4,965)
Net operating income (loss)		1,037	1,176
Corporate [Member]
Segment Reporting Information [Line Items]
Total revenue		1,003	1,310
Operating expenses	[1]	(1,799)	(1,966)
Net operating income (loss)		$ (796)	$ (656)

[1] Includes property operating expenses, real estate taxes and third party management expense.

[2] On December 11, 2018, the Company acquired from DRA Advisors its 50% ownership interest in the G&I Austin Office LLC real estate venture. The DRA Austin Venture owned twelve office properties containing an aggregate 1,570,123 square feet, located in Austin, Texas. As a result of the acquisition, the Company acquired complete ownership of the Austin properties.

[3] On December 20, 2018, the Company contributed a portfolio of eight properties containing an aggregate of 1,293,197 square feet, located in its Metropolitan Washington, D.C. segment, known as the Rockpoint Portfolio, to the Herndon Innovation Center Venture. The Company and its partner own 15% and 85% interests in the Herndon Innovation Center Venture, respectively.